1. Field of the Invention
The present invention relates to switched power supplies for laser systems and, more particularly, to a high speed switching power supply for a light controlled laser system.
2. Prior Art
Gas-filled lasers, as for example, argon, krypton, neon or helium type lasers, as well as solid-state lasers that are pumped with gas-filled flash tubes, are typically energized by power supplies that regulate the power of the laser output light. In the past such power supplies have achieved regulation of the laser light power by using a linear pass element to control the electrical current allowed to flow to the laser tube. Typically, the linear pass element consists of a bipolar transistor which functions as a variable current source connected in series between a laser tube and a source of constant d.c. voltage. The linear pass element is controlled by a feedback control circuit which uses the laser light power level of the laser beam current as a feedback parameter.
The problem with utilizing a linear pass element for purposes of controlling laser light power is that the linear pass element dissipates a relatively large amount of energy. For example, the unregulated voltage derived from the a.c. power mains is typically 20% to 30% higher than the laser operating voltage. The difference is taken up by the linear pass element so that 20% to 30% of the power from the a.c. power mains is dissipated by the linear pass element. This large power loss requires a complicated cooling system as well as reducing the efficiency of the laser system.
Some attempts have been made in the prior art to reduce the large power loss that is attendant with the use of a linear pass element for controlling laser beam power. See, for example, U.S. Pat. No. 4,017,745 to McMahan. The McMahan patent discloses a switched power supply wherein parallel-connected transistors, disposed in series between a fixed d.c. voltage and the load, are alternately switched on and off. By varying the duty cycle of the switching transistors, the voltage across a load-bridging capacitor is controlled. When used in conjunction with a laser type of load and a linear pass element, the power dissipation across the linear pass element is reduced to something on the order of 5% of the power dissipated by the laser tube by reducing the amount of the voltage dropped across the linear pass element. Although this greatly improves the overall efficiency of the laser system by reducing the power dissipated through the linear pass element, nevertheless, the increased efficiency comes at the cost of a more complex power supply. Thus, what is needed is a light controlled laser power supply which operates entirely in the switching mode without the aid of the linear pass element employed in virtually all types of light controlled laser systems currently in use. Such a power supply would be more compact and thus more economical to manufacture. However, until the present invention, such a power supply has not been available because of several problems.
For example, the open loop gain function has a frequency dependent magnitude component and a frequency dependent phase component which describe the electrical gain in the control loop of the power supply. The system stability criterion first stated by H. Nyquist in 1932 is that the magnitude of the open loop gain function must be less than unity at those frequencies for which the phase component is greater than 180 degrees. This stability criterion places an upper limit on the system control bandwidth. That is, the system gain function must be rolled off to less than unity before the frequency is reached at which the phase shift accumulation in the system has exceeded 180 degrees. Otherwise, undesirable loop oscillations will occur.
The open loop gain function of the switching type power supply is inherently different from that of the linear pass type power supply. The linear pass element is a high impedance current source and hence, the laser light feedback analog signal directly controls tube current. On the other hand, a switching regulator type power supply typically has a low impedance voltage output so that in this system, the laser light feedback analog signal controls the laser tube voltage rather than current. In a plasma type laser tube, a 5% variation in voltage about some operating point will produce a 50% variation in tube current. This means that the open loop gain function of the switching regulator type power supply is 10 times greater in magnitude than the linear pass type power supply. Consequently, it is much more difficult to accurately control the laser light output without causing system oscillation.
Additionally, the switching regulator type power supply contains a reactive energy storage circuit that restores the switched voltages and currents to the d.c. output level. The energy storage circuit consists of an inductor and a capacitor that cause a 180 degree phase lag at their resonant frequency. The resonant frequency of the reactive storage circuit is set by the switching frequency of the switching controller.
The bandwidth of a switching regulator type laser power supply is limited by the phase lag that occurs as the frequency approaches the resonant frequency of the reactive storage elements. The bandwidth problem is further complicated by the large inherent gain of the low impedance output of the switching type regulator, since the extra gain must be compensated for by a more severe gain roll off to achieve less than unity gain at the resonant frequency of the reactive storage elements.
Thus, until the present invention, the foregoing problems have not been satisfactorily resolved, and this has resulted in the continued use of the linear pass element, with its attendant disadvantages, in virtually all types of laser light controlled power supply systems currently in use. The light controlled laser power supply of this invention overcomes these problems. We have shown experimentally that a unity gain bandwidth of at least 1000 Hz must exist in the low impedance switching type regulator in order to achieve a commercially acceptable laser beam noise level of less than 5 percent peak to peak referred to the beam's average intensity. In order to open up the bandwidth to 1000 Hz, the switching frequency needs to be increased to 100 KHz or more. This allows the resonant frequency of the reactive storage elements to be greater than 2 KHz. Also, to provide acceptable stability as the unity gain of the magnitude function approaches the LC resonant frequency, rate feedback needs to be employed whose phase lead properties tend to cancel the phase lag of the LC resonant circuit.